As discussed in detail in U.S. Pat. Nos. 4,312,842, 4,415,543 and in co-pending application Ser. No. 634,965 (the substances of which are herein incorporated by reference), this invention relates to the wet process or acid process of making phosphoric acid, and particularly relates to the removal of fluorine and phosphorus values from the treatment of the waste phospho-gypsum. The gas stream from phospho-gypsum treatment is free of fluorine to a level which makes the gas acceptable for use with a vanadium oxide catalyst (or other type catalyst) in a contact sulfuric acid plant and the gas has a composition which is desired for efficient operation of the sulfuric acid plant.
The prior art is succinctly described in U.S. Pat. No. 4,312,842 and in co-pending U.S. Pat. No. 4,415,543 and Ser. No. 634,965. The present application is different from and in certain aspects an improvement in these processes in that the fluorine values are removed by treating in a lime or limestone scrubber where the fluorine containing material, including hydrofluoric acid, is converted on the surface of the limestone to fluorspar.
The principal objective of this invention is a process whereby wet phospho-gypsum, the by-product of wet phosphoric acid manufacture, is treated to (a) remove water, acids, and the fluorides in the liquid phase from the phospho-gypsum by drying to about 400 to 600 deg. C., (b) calcine the phospho-gypsum at about 1225 deg. C. to dissociate the lime from the sulfur trioxide, (c) cool and store the siliceous lime (d) effect further fluoride removal from the gases by treatment in lime or limestone, (e) dry the gases in sulfuric acid, and (f) use the resulting gas stream to manufacture sulfuric acid. In this manner, all of the waste phospho-gypsum is recovered as useful products.
As shown in the accompanying drawing, the wet phospho-gypsum is first dried in a fluid bed dryer to 400 to 600 deg. C. The gaseous stream from the dryer is first scrubbed by lime/limestone to remove the fluorides. Virtually all of the volatile fluorides originally in the phospho-gypsum are removed in this step. The hot gases are then used to generate steam for use in the processes. Finally, they are treated, if necessary, and vented. The dried solids from the dryer are sent to a calciner unit.
The calciner is a fluid bed in which a source of carbon is added to the phospho-gypsum and the mixture heated to 1225 deg. C. under reducing conditions. In the final stage, excess air is added to eliminate all undesirable by-product gases, leaving only sulfur dioxide, oxygen, carbon dioxide, and nitrogen. These gases are lime/limestone scrubbed and then sulfuric acid scrubbed to remove any remaining fluoride gases and to dry the gas stream. The gas stream is then suitable for delivery and use in a modern sulfuric acid plant. These and other objects and advantages will become apparent hereinafter.
The invention also consists in the parts and in the arrangements and combinations of parts hereinafter described and claimed.